If you have several IP addresses for use over your OpenVPN tunnel in windows you need to set them as an alias IP address on your TAP (tunnel) interface.

On the netword card TAP-Win32 Adapter V8, go to TCP/IP-settings, on the first screen make sure you use a static and not an automatic IP address.Fill in your first IP-address there. For example 88.80.28.x. Next, click the Advanced button and add all your other IP addresses there.

To actually be able to use the additional IP-addresses with a program, the program itself will have to support binding on another IP address in order for it to work.